The optical transmission systems currently on the market make use of the so-called “wavelength division multiplexing” (WDM) technology that allows transmission bit rates of 10 Gbps, 40 Gbps, even 100 Gbps, by using the 50 GHz ITU grid, in other words, a grid in which the spacing between two WDM optical channels is 50 GHz (or 0.4 nm).
The long haul optical transport networks have progressively adopted a meshed architecture, through the implementation of reconfigurable wavelength optical add/drop multiplexers, also called ROADM. These ROADM multiplexers make it possible to transparently route the WDM channels without the need for costly optical/electrical/optical (OEO) regeneration operations.
With the rise in bit rate and the appearance of 400 Gbps and 1 Tbps “super-channels”, it is becoming possible to optically disaggregate the constituent optical sub-bands of these “super-channels”. As a result, a new so-called “orthogonal frequency division multiplexing” (OFDM) technology can be employed, within these very constituent sub-bands, this OFDM technology being more robust than the WDM technology compared to the chromatic and modal dispersions of polarisation, offering a rectangular spectrum that is more tolerant in optical filtering terms, while further allowing for the use of different modulations according to the sub-carriers.
With this in mind, a new grid has therefore been standardized by the ITU-T (standard G.694.1) in order to offer a framework for the implementation of these “super-channels”. In this new grid, the sub-wavelengths (at the centre of the optical sub-bands) have to be aligned on a grid with a pitch of 6.25 GHz, whereas the granularity of the spectral slot is 12.5 GHz.
It follows that from the optical routing of the constituent optical sub-bands of “super-channels” within the future entail the use of ROADM multiplexers that are more selective and narrower than those currently used on the traditional WDM transmission systems, the spectral transfer function of these optical multiplexers having to be as rectangular as possible.
Now, the perfectly rectangular nature of the narrow spectral transfer function of the optical filters, in particular of the sub-band multiplexers, cannot be guaranteed. As a result, the cascading of a plurality of sub-band optical filters or multiplexers, for example in a cascade of ROADM multiplexers, necessarily degrades the initial form of the spectrum of the constituent OFDM sub-bands of a “super-channel” in the transmission of this “super-channel” through these filters.
FIGS. 1A and 1B thus illustrate the distortion created on the spectrum of an OFDM optical signal of 8 GHz bandwidth, here comprising 256 sub-carriers and suitable for acting as a constituent optical sub-band of a “super-channel” as discussed previously, these figures illustrating the spectrum of this OFDM signal respectively before and after a cascade of 5 sub-band dropping selective optical filters.
It can be seen that, although the initial spectrum is substantially flat in FIG. 1A, the difference in power between the sub-carriers in mid-spectrum and those at the spectrum edge is approximately 10 dB after passing through the optical filters, in FIG. 1B.
The performance in reception of such an OFDM optical signal is greatly degraded, in that the different constituent sub-bands of this OFDM signal then exhibit extremely variable bit error ratios (BER), the signal-to-noise ratio (SNR) between these different sub-carriers no longer being at all the same at the receiver of this OFDM signal.